A common strategy for achieving high availability and fault tolerance in data-processing systems is to employ a primary system and a backup system (or a plurality of backup systems), and to duplicate (or shadow) the primary system's state onto the backup machine(s), thereby enabling near-seamless failover to the backup if the primary fails. Unfortunately, the additional overhead required to maintain a backup system and keep its state in synch with the primary system can significantly degrade performance. What is needed, therefore, is a technique for improving performance in high-availability systems.